1. Field of the Invention
The present invention relates to transmitting digital data over analog channels.
2. Description of Related Art
In many communication systems, a modem is used to transmit digital data. The modem converts the data into a sequence of signal points and transmits the signal points to a near-end central office via an analog channel. In the near-end central office, a pulse code modulation voice coder (PCM vocoder) maps the received signal points to a set of quantization levels. The near-end central office transmits the sequence of quantization levels upstream to a far-end transceiver via a digital network.
In the analog channel, continuous time signals broaden and overlap through a phenomenon referred to as inter-symbol interference (ISI). To remove ISI, conventional modems precode the signals prior to transmission to the analog channel. After precoding, channel broadening does not produce ISI. Instead, channel broadening converts signals that represented signal points of an original constellation prior to precoding into signals that represent signal points of an extended signal constellation. Conventionally, both original and extended constellations are derived from the quantization levels of the PCM vocoder. Precoding based on such constellations provides good noise performance in the absence of channel noise and echo.
Quantization replaces the continuous values of signals received from the analog channel by discrete levels through a process that includes sampling and either truncation or rounding of the received signals. Since quantization replaces continuous values by discrete levels, quantization introduces noise into the transmitted data. The magnitude of this quantization noise depends on the distance between the quantization levels of the PCM vocoder.
The far-end transceiver transmits signal points downstream to the near-end central office via the digital network. In the near-end central office, the signal points are converted into continuous time signals that are transmitted downstream to the modem via the same analog channel that carries other data upstream. The downstream transmission does not generate quantization noise if the values of signal points from the far-end transceiver correspond to quantization levels in the near-end central office. Quantization noise does not occur, because the digital network does not distort signal points. Thus, the near-end central office receives from the far-end transceiver signal points that correspond to the quantization levels.